In semiconductor fabrication, there is a requirement, in various machines and measuring instruments, to move a wafer along a vertical Z-direction without rotating it about the Z-axis during this movement. This direction of rotation will here be denoted as “rZ.” Thus, in an inspection device for quality control, a wafer may be placed in a pre-adjusted position on a stage, which is then moved vertically into the focal range of a microscope. To be able to inspect the regions of interest of the wafer, the orientation of the wafer must be maintained, except for the Z-direction. Rotation in rZ would be particularly disadvantageous because it would have the effect of a large displacement of the searched position in the plane of the wafer in the peripheral regions of the wafer; i.e., far away from the center of rotation. It is therefore desired to have a Z-axis having as small an angular error as possible.
A straight-line mechanism which is adjustable without play is known from EP 0816013 B1. Here, a wedge-profiled element between a guide shaft and a bearing bush allows the play between the guide shaft and the bearing bush to be adjusted by rotating the bearing bush relative to the guide shaft. The bearing bush is attached to the slider by means of oblong holes, so that the play can be readjusted when the shape of the bearing bush has changed due to wear. By providing three wedges around the circumference of the bearing bush and guide shaft, the two elements remain centered relative to each other. Furthermore, it is proposed to tangentially preload the bearing bush by means of a spring, so that a play-free yet easy sliding is possible at all times, even when wear has occurred or under thermal expansion conditions. However, this straight-line mechanism still allows a certain rotation of the slider relative to the guide shaft since the wedge-profiled elements are all oriented in the same direction and have only a small slope angle. Therefore, the bearing bush and slider may rotate about the guide shaft, and especially easily in one of the two directions of rotation because the wedges disengage from one another in this direction, thereby increasing the play of the straight-line mechanism. In order to move a movable part linearly and block its rotation about the linear direction, at least two such straight-line mechanisms are required here.